The present invention relates to a method of and a device for simultaneously measuring a refractive index and a thickness of a medium, a method of and a device for measuring a birefringence of a medium, a method of and a device for simultaneously measuring a birefringence and a thickness of a medium, a method of and a device for simultaneously measuring a phase index and a group index of a medium, and a method of evaluating a curing condition or a degree of hardness of hardenable resin with the use of these methods and devices.
A method of measuring an optical characteristic such as a refractive index n (=phase index np), a birefringence, a thickness or the like of medium in a noncontact manner, is one of most basic technology in the optical field. The most typical one is a method which utilizes an elipsometer (automatic polarization analyzing device refer to Polarization Analysis, 2, 4, pages 256 to 264 of "Optic Handbook" edited by Hiroshi Kobota, issued by Asakura Book Store), and which is carried out by measuring a difference between phase variations of P-polarization and S-polarization of reflecting light obtained by projecting light obliquely to a medium (thin film) to be measured, that is, a polarizing condition of light reflected on the surface of a medium is observed so as to measure a refractive index n and the thickness t of a substrate or a thin film deposited on the surface thereof. Further, as disclosed in, for example, Japanese Patent Laid-Open No. 64-75902, Japanese Patent Laid-Open No. 63-128210 and Japanese Patent Laid-Open No. 3-17505, a reflection analyzing method for measuring a film thickness, a refractive index n and an absorption coefficient of a medium with the use of a reflectance factor has been also used. The reflectance analyzing method disclosed in the Japanese Patent Laid-Open No. 64-75902 or the Japanese Patent Laid-Open No. 63-128210 measures a variation in the intensity of reflecting light which is caused by a variation in incident angle of a measuring light beam, that is, an incident angle depending characteristic of the reflecting light, and uses three extreme values of incident angles so as to obtain a characteristic value of a thin film. A reflectance analyzing method disclosed in the Japanese Laid-Open Patent No. 3-17505 obtains a characteristic value of a thin film from an incident angle depending characteristic of a reflected light intensity obtained by detecting a light intensity in the rear of a detecting lens.
Apparatuses for carrying out the above-mentioned methods must have a high degree of accuracy, and accordingly are prosperously used for studying surfaces or thin films. However, these apparatuses themselves are expensive, and further, can merely measure an average refractive index n and thickness t in a part (having a diameter of 1 mm) irradiated with a collimated beam. Moreover, the thickness which can be actually measured is up to about 10 .mu.m, that is, a thickness exceeding this value cannot be measured. In addition to the above-mentioned methods, methods of measuring a refractive index of a medium with the use of a prism, a refractive index n and a thickness t of a thin film through light guide mode excitation, are also used, but the use of these methods is limited by such a condition that a surface to be measured is flat and smooth. There has been a such a demand that a refractive index n (including a birefringence)and a thickness t of a medium including inorganic and organic materials, and a spatial distribution thereof, should precisely be measured, in methods of mainly measuring a thin film in the optical field. Further, at present, a gel fraction (a ratio between an initial weight of a sample to be measured and a variation in the weight of the sample after a solvent is extracted by using methyl-ethyl ketone or the like) is most typically used as an index for evaluating several kinds of hardenable resin (such as those which are ultraviolet-hardenable, thermo-hardenable, catalyst-hardenable, and electron-beam hardenable). However, such an evaluation uses a destruction test which requires a long time for preparation and valuation of a sample.
In view of the above-mentioned circumstances, the present invention proposes the following methods, basically using an interference optical system including a low coherence light source: a method of precisely and simultaneously measuring a phase index np and a thickness t of a medium through irradiation of a focused light beam, a method of precisely measuring a birefringence of a medium without the necessity of polarization control of a polarizer, an analyzer, a polarizing rotator or the like, and a method of precisely and simultaneously measuring a phase index np and a group index ng, and as well a method of evaluating a hardenability or a hardness of resin with the use of the above-mentioned methods.
A Michelson interferometer using a low coherence light source which can recognize a reflecting surface along a light propagation axis with a resolution (up to 10 .mu.m) which is determined by a coherence length 2.DELTA.lc (=ln(2)(2/.pi.)(.lambda.c.sup.2 /.DELTA..lambda.), where .lambda.c is a center wavelength of a light source, .DELTA..lambda. is a full width at half maximum (FWHM) of a spectrum of a light source), has been used as an effective diagnosing method in a microarea (refer to (2): "New Measurement System for Fualt Location In Optical Waveguide Devices based on an Interferometric Technique" by K. Takada, I. Yokohama, K, Chida and J. Noda, Applied Optics, 1087, Vol. 26, No. 9. Pages 1,603 to 1,606; (3): "Optical Coherence-Domain Reflectometry; a New Optical Evaluation Technique" by R. C. Youngquist, S. Carr and D. E. N. Davies, Optics Letters, Vol. 12, 1987, No. 3, pages 158 to 160; and (4): "Submillimeter Optical Refletometory" by H. H. Gilgen, R. P. Novak, R. P. Salathe, and W. Hodel and P. Beaud, J. Lightwave Technology, vol. 7. 1989, No. 8, pages 1,225 to 1,233).
Recently, even in the field of biological optical diagnosis, the above-mentioned low coherence optical interferometry has attracted great attention. With the use of this technology, there have been progressed detection and visualization of a tissue underneath the retina (refer to (5) "Optical Coherence Tomography, by D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Chang, M. R. Hee, T. Flotted, K, Gregory, C. A. Puliafito and J. G. Fujimoto, Science, Vol., 254, pages 1,178 to 1,181; and (6): "Optical Coherency Microscopy in Scattering Media" by J. A. Izatt, M. R. Hee, G. M. Owen, E. A. Swanson and J. G. Fujimoto, Optics Letters, vol., 19, 1994, No. 8, pages 590 nto 592), measurement of an eye length ((7): "Eye-Length Measurement by Interferometry with Partially Coherent Light" by A. F. Frecher, K. Mengehot and W. Wrener, Optics Letters, Vol. 13, 1988, No. 3, pages 186 to 188, and (8); "Measurement of the Thickness of Fundus Layers by Partial Coherence tomography" by W. Drexier, C. K. Hltzenberger, H. Sattmann and A. F. Frecher, Optical Engineering, Vol. 34, 1955, No. 3, pages 701 to 710), and a basical experiment for precise detection of subcutaneous tissue ("Basical Experiment I for Precise Detection of Subcutaneous Tissue" by Shiraishi, Ohmi, Haruna and Nishihara, 56-th Scientific Lectured by Applied Physics Association. Autumn 1995, 26a-SN-11).
However, in the above-mentioned low coherence interferometry, collimated or focused beam is irradiated onto an object (a transparent plate) to be measured while the positions of two reference light mirrors are specified so that an optical path difference between reflecting signal beam and reference light from the front and rear surfaces of the object becomes zero, and an optical path length (group index ng.times.thickness t) between the front and rear surfaces of the object is measured from the spatial distance between the two mirrors. That is, since only the value of a group index.times.a thickness t) can be measured in this case, the group index or the phase index and the thickness t cannot separately be measured. It is noted here that the group index ng is dependent upon the FWHM of a light source, that is, the larger the FWHM, the larger the affection of wavelength dispersion, and accordingly, .DELTA.n (=ng-np) becomes larger. It is noted that the refractive index n so-called in general is the phase index np.
The measurements of a phase index np (including a birefringence) and a thickness of a medium are indispensable factors for manufacturers who developing optical parts including lenses and optical materials. In particular, as to lenses, the measurement of a precise distribution of thickness is required together with a phase index np. Recently, in addition to optical parts made of various multi-component glass materials, optical parts made of polymers or liquid crystal, are widely used, the technology of and devices for simultaneously and precisely measuring a phase index np (including a birefringence) and a thickness of the material are indispensable for developing these parts. Further, the research and development for various nonlinear optical materials are prosperous at present, in order to use a short wave light source or a variable wavelength laser, and accordingly, the measurement of the refractive index (including a birefringence) of such a new optical material requires a convenient apparatus for simultaneously measuring a phase index np and a thickness t.
Further, in the medical field including the optical diagnosis and treatment field, the necessity of simultaneous measurements of a phase index and a thickness has been becoming hither and higher. For an example, ophthalmic diagnosis and treatment require precise measurements (with a high degree of precision which is about 10 .mu.m) of an eye length, a thickness and a refractive index of the cornea. In this case, noncontact measurement is required as an indispensable condition, and accordingly, an optical probe is used. However, at present, since the refractive index ng and the thickness t cannot be measured separately from each other, the eye length, the thickness and the refractive index of the cornea cannot be precisely measured. Further, even with an optical CT (the optical build-up of optical biological tomographic images) which has been eagerly studied, simultaneous measurements of a refractive index np or ng and a thickness t are required.
Further, as to the quality evaluation and management of several kinds of protecting layers (films), evaluation using a refractive index as an evaluating index has bright prospects. This is directed to such a fact that the refractive indices of various kinds of hardenable resin vary due to shrinkage or the like as it is hardened. If a refractive index alone is measured, it can be made by using a usually used Abbe's refractomenter. However, such an Abbe's refractometer can measure a refractive index of a part only at a surface of an object to be measured, but cannot measure an averaged refractive index of an object to be measured, having a nonuniform distribution of refractive index in the thickness wise direction thereof (the surface of the object to be measured has been hardened but the interior thereof has been not yet, and so forth). This fact offers a serious barrier to evaluation of a hardened condition or a hardness of a resin, using refractive indices as evaluation indices. Further, a sample piece obtained from an object to be measured has to be made so as to satisfy terms required by an Abbe's refractometer, and a nondestructive measurement and a noncontact measurement cannot be carried out. In the case of using ultraviolet hardenable resin as an object to be measured, a hardening condition varies depending upon a thickness of the object to be measured, and accordingly, the value relating to the thickness of the object to be measured must be measured.
The present invention is devised in view of the above-mentioned circumstances, and accordingly, one object of the present invention is to provide a method of and an apparatus for measuring a medium, which can measure a phase index np, a birefringence and a thickness t of an object to be measured, separately from one another, and which uses an optical interferometry that can simultaneously measure both phase index np and group index ng, and to provide a method of evaluating and measuring a hardened condition or a hardness of hardenable resin with the use of a refractive index and with the use of the above-mentioned method and apparatus.